Conventionally, one such ventilation apparatus for an engine is described in, for example, Patent Document 1. Generally, conventional ventilation apparatuses for an engine, including the apparatus described in Patent Document 1, ensure ventilation in a crank case by drawing and recirculating blow-by gas from the interior of the crank case, into which the blow-by gas has leaked via the clearance between a piston and a cylinder, to the intake passage through a recirculation passage. In the conventional ventilation apparatuses, a recirculation passage has an opening at the end corresponding to the crank case and an opening at the end corresponding to the intake passage but does not include any other opening communicating with the exterior such that the blow-by gas cannot leak from the recirculation passage.
Blow-by gas contains oil mist, which adheres to the inner wall of the intake passage if recirculated to the intake passage together with the blow-by gas. To avoid this, the technique described in Patent Document 1 has an oil retaining groove (2), which is formed in the recirculation passage. Blow-by gas thus passes through oil retained in the oil retaining groove (2) such that oil mist is removed from the blow-by gas (see FIG. 1 of Patent Document 1).